Effect of Bi Content on Properties of Low Silver SAC Solders SMTAI FINAL

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EFFECT OF Bi CONTENT ON PROPERTIES OF LOW SILVER
SAC SOLDERS
Mehran Maalekian, Ph.D., Yuan Xu, Karl Seelig*
AIM Metals & Alloys
Montreal, Canada / Cranston, RI, USA*
[email protected]
ABSTRACT
Since the adoption of RoHS and REACH regulations for
electronics manufacturers, Sn-Ag-Cu (SAC) alloy systems
have largely replaced the Sn-Pb solder alloys with Sn-3Ag0.5Cu (wt%) (SAC305) the most widely accepted. This
high silver content SAC alloy (greater than X percent) has a
major deficiency however, the relative fragility of the solder
joint. This is due to the formation of primary Ag3Sn
platelets crystals in the solidified structure leading to poor
drop-shock performance causing major concerns for
portable devices. Lowering or eliminating Ag content of
SAC has been proposed as a solution but with limited
success. In this work, a systematic study is presented to
address the effect of Bi content on soldering and mechanical
performance of low silver SAC alloys. Thermal behavior
(melting and solidification), wetting and spreading
performance, and tensile and hardness properties of alloys
are compared and a promising lead-free SAC-Bi solder
alloy that also demonstrates tin whisker mitigation
properties is recommended.
Key words: Lead-free solder, electronic assembly,
wettability, mechanical properties, melting, tin whisker
INTRODUCTION
Soldering is the most important part of assembly process of
electronic components. Among many various criteria for
creating a robust solder joint are wettability, spreading,
thermal behavior (melting and solidification), mechanical
properties such as tensile strength and creep resistance,
thermal and electrical conductivity and cost. Prior to RoHS,
Sn-Pb alloys had been widely used as the main solder
material for electronic packaging due to its low cost,
relatively low melting temperature, superior wetting
behavior, high reliability and relatively good mechanical
performance. However, due to environmental and health
concerns from the toxicity of lead (Pb), it has been banned
for use in electronic and electrical devices, resulting in
extensive study to find a suitable replacement. Fig. 1
illustrates how rapidly the global solder industry responded
to the demand for lead-free solders in many electronic
applications. Among the lead-free alternatives, Sn-Ag-Cu
(SAC) alloys are regarded as the de facto replacement for
Sn-Pb solders. The most popular of these alloys is SAC305
containing 3% silver, 0.5% copper and the balance (96.5%)
tin (wt.%).
The relatively high silver content of SAC305 introduces two
major concerns related to performance and cost. SAC305 is
prone to drop-shock and creep failure due to the formation
of large Ag3Sn platelets and poor creep resistance [6,7].
SAC alloys exhibit high undercooling that is associated with
the difficulty of nucleating Sn solidification as a proeutectic phase. Thus, during slow cooling, e.g., in ball grid
array (BGA) joints where cooling rates are slow (<0.2°C/s),
the high undercooling of the joints can promote formation of
undesirable pro-eutectic intermetallic phases, particularly
Ag3Sn blade-like phase, that tend to coarsen drastically
making the as-solidified solder joints more brittle [6]. In
addition, the price of silver has increased drastically over
the last several years driving the industry for lower silver or
lower cost solder alloys.
The aim of this study is to examine a low silver content
alloy to reduce the solder cost and improve mechanical
performance by elimination of primary large Ag3Sn
platelets. However, by lowering Ag content of SAC alloy,
melting temperature will increase. The addition of bismuth
(Bi) as a low cost element can reduce the melting
temperature and improve mechanical properties. Bi may
also reduce mitigate tin whisker growth which is very
important in the context of lead-free solders in high
reliability electronic assembly. Thus, in this work binary Sn0.7Cu is taken as the base solder alloy. The presence of
copper (Cu) reduces the copper dissolution rate from the
terminal pad. The silver (Ag) content of the alloy is kept
below 1% and the added Bi from low (0.5%) to high (22%)
values. Soldering and mechanical performance of the alloys
are studied and compared.
Pb-free % of global shipments
80
60
40
20
0
2003
2005
2007
2010
Figure 1. Global consumption of lead-free versus tin/lead
solders (in wt%). In 2010, for example, worldwide
consumption of lead free solder was at 70%, whereas it was
30% for tin/lead [5].
EXPERIMENTAL
Twelve alloys with varying Bi content (Table 1) were
prepared in an electric pot with graphite crucible. The
composition of solder alloys was analyzed by arc emission
spectroscopy. The molten alloys were cast into round ‘dogbone’ shape in a stainless steel mold for tensile testing. The
tensile specimens with the dimensions shown (Fig.2) were
annealed at 125°C for 96 hr. to remove any residual stress or
inhomogeneity induced during the sample preparation.
Tensile tests were performed at room temperature with a
constant strain rate of 10-3 s-1. For each alloy at least five
specimens were tested to ensure reproducibility of the
tensile test results. Hardness of all the specimens was also
measured by Vickers micro-hardness testing machine with
10gf load and 10s impression time.
Wettability is a critical factor when evaluating the
performance of a solder alloy. The wetting process is
dynamic during soldering and the solid/liquid interfacial
tension between solder and substrate varies as the wetting
proceeds. Wettability can be quantified with wetting balance
technique that measures wetting force between molten
solder and solid substrate versus wetting time. Wettability of
the alloys was therefore quantified with wetting balance
technique. A copper plate coupon (25mm×6mm×0.5mm)
used as the substrate was immersed in a bath of liquid solder
alloy and the wetting force was measured over time. The
wetting balance curve displays the vertical component of
force (F) applied by the wetting balance load cell on copper
coupon (see Fig. 3). This technique will provide real-time
wetting force that can be evaluated as wetting performance
of solders and fluxes. In the typical wetting balance curve
shown in Fig. 3b, ‘t0’ is the time for the wetting curve to
recross the buoyancy force line and the contact angle falls to
ϴ =90°, this time is called zero cross time. In this study, the
time ‘t0’ and the maximum force are taken as wetting time
and wetting force, respectively. An alcohol based no clean
flux (AIM NC 265) was used to flux/prepare the substrate
surface. Each test was repeated three times and the average
value was reported. Details of wetting balance test condition
are shown in Table 2.
Table 1. Compositions of solder alloys (wt%)
Sn
Cu
Ag
Bi
Bal.
0.7
0.6
0-22
Thermal behavior of the alloys was investigated with
differential scanning calorimetry (DSC). A piece of 50100mg alloy was heated in an aluminum cell at heating rate
of 10°C/min under nitrogen atmosphere followed by cooling
to room temperature at -10°C/min cooling rate. Each alloy
was tested twice and the reading was based on the second
run.
Figure 2. Dimensions of round bar tensile specimen (dogbone).
(a)
(b)
Figure 3. (a) Wetting balance test running on Cu coupon
immersed in molten solder bath. Wetting force (F), contact
angle (ϴ), surface tensions at the solid-liquid (γSL), solidvapor (γSv) and liquid-vapor (γLv) interfaces are
schematically shown. (b) Typical wetting curve shows the
wetting balance force versus wetting time.
Table 2. Wetting balance test parameters
Molten alloy temperature
265°C
Pre-heat time
However, the solidus and liquidus parallel lines start to
deviate at higher Bi content as depicted with dashed line at
3% Bi in Fig. 6. Pasty range which is defined as the
difference between solidus and liquidus temperatures (see
Fig. 7) increases slightly with Bi content up to 3 %;
however, the increase is drastic above 3% as depicted in
Fig. 7.
10 sec
Immersion speed
20 mm/s
Immersion depth
6 mm
Immersion time
5 sec
In order to evaluate tin whisker growth, a 2mm diameter
copper wire was fluxed and dipped into the molten solder
pot followed with wiping in order to give a uniform coating
thickness. The coated wire was then stressed by making a
“U-shaped” bend to 90 degrees. These samples were
exposed to an accelerated whisker growth condition, i.e. in
an environment chamber with 60°C temperature and 85%
relative humidity. The surfaces of samples were regularly
examined with scanning electron microscope (SEM) for tin
whisker growth.
RESULTS AND DISCUSSION
Thermal behavior
DSC heating curves (heat flow versus temperature) are
shown in Fig. 4. The valleys represent intense endothermic
phase transitions, i.e. melting. Based on thermal behavior of
the alloys DSC diagrams are separated in two plots in Fig.
4: the top diagram for relatively low Bi alloys and bottom
diagram for high Bi alloys. DSC results clearly show that
high Bi content SAC-Bi alloys that form very low melting
temperature phase (circled) and are not suitable for further
consideration. Thus, the focus of this work was directed to
lower Bi content alloys.
In order to better understand effect of Bi on the thermal
behavior of the alloys, a thermodynamic calculation based
on Scheil-Gulliver solidification model was carried out
using the FactSage™ thermodynamic software [8]. The
calculated weight percent of liquid phase in each alloy as a
function of temperature is illustrated in Fig. 5. The
solidification model which is partial equilibrium approach
(full diffusion of solute in liquid but no diffusion in solid)
shows that the low melting temperature phase is significant
(>15%) in alloys with Bi content of 10% and above.
However, the low amount of liquid phase solidified at low
temperature (~140°C) in low Bi alloys (Bi<6%) explains the
absence of detectable low temperature peak in DSC curves.
For solidus determination of the alloys the temperature of
the first visible onset of melting has been taken. In other
words, as soon as a deflection from the baseline of DSC
curve occurs, melting is taking place. For liquidus the peak
temperature is taken. Fig. 6 depicts that the solidus and
liquidus of SAC-Bi alloys decrease linearly in parallel
fashion with increasing Bi content up to about 3wt% Bi.
Figure 4. Heating curves (heat flow vs. temperature) of the
alloys at the heating rate 10°C/min: top diagram shows low
Bi (Bi<6%) content SAC0607-Bi alloys and bottom diagram
shows high Bi (>9%) content SAC0607-Bi alloys with low
melting phase peak.
Mechanical properties
Tensile stress-strain curves of the alloys (Fig. 8) show that
by increasing Bi content the strength of SAC-Bi alloys
increases while ductility decreases. Bi bearing alloys all
show higher strength as compared to that of SAC305. Fig. 9
shows that the increase of tensile strength with Bi content is
linear. However, elongation decreases with addition of Bi
content in a non-linear fashion and follows a power law
function up to 3% Bi as shown in Fig. 10. The elongation of
6% Bi alloy falls below the power law fitting equation
indicating a rapid reduction in ductility of the alloy.
Furthermore, consistent with tensile strength results,
hardness data (Fig. 11) also demonstrates that the hardness
of alloys increases linearly with Bi content.
Figure 5. Calculated liquid fraction vs. temperature of
SAC-Bi alloys at different Bi contents.
Solid solution of Bi in Sn is the main strengthening
mechanism [9]. Precipitation of Bi in β-Sn matrix at higher
Bi content may also contribute in strengthening of the
alloys. A thermodynamic calculation of equilibrium phases
illustrated in Fig. 12 confirms that all Bi dissolves in the
matrix at Bi concentrations below 3wt%. Therefore, the
only strengthening mechanism is solid solution of Bi in βSn. Since Ag and Cu contents are kept constant,
concentrations of intermetallic phases, i.e. Ag3Sn (0.8 wt%)
and Cu6Sn5 (1.8 wt%) remain identical for all the alloys.
The equilibrium calculation also shows that when Bi content
is above 3 wt% the concentration of Bi particles increases as
a result of limited solubility of Bi in β-Sn. Thus,
precipitation hardening mechanism may also contribute in
mechanical properties of the alloys.
Figure 6. Solidus and liquidus temperatures of the alloys
determined by the first deflection of the baseline in DSC
curve. Parallel solidus and liquidus lines deviate at 3% Bi.
Figure 8. Tensile stress-strain curves of
specimens. SAC305 is shown for comparison.
Figure 7. Variation of pasty range of the SAC-Bi alloys
with Bi content.
annealed
Figure 9. Tensile strength of SAC-Bi alloys changes
linearly with Bi content.
Figure 10. Tensile elongation of the alloys in the annealed
condition. Up to 3% Bi the elongation fits with a power law
equation. Elongation decreases more rapidly in alloy with
higher concentration of Bi (6%).
Figure 11. Hardness of the alloys in the annealed condition.
Figure 12. Equilibrium calculated wt% of Bi phase
precipitated in β-Sn at 25 ⁰C for the alloys Sn-0.6Ag-0.7CuXBi. Other phases calculated for the alloys are: 0.8 wt%
Ag3Sn, 1.8wt% Cu6Sn5 with balance of β-Sn.
Wetting behavior
The wetting balance test results are shown in Fig. 13. It is
found that bismuth addition above 2wt% improves
wettability of SAC-Bi alloys by increasing wetting force
and reducing wetting time as demonstrated with the shifting
wetting curve towards top left of SAC0607 curve. Graphs of
wetting time (t0) and wetting force as function of Bi content
are shown in Fig.14. Fig.14a clearly shows that Bi addition
increases the wetting speed. The trend is captured with a
single polynomial function. The fact that addition of Bi
reduces the melting temperature of the alloys (see Fig. 6)
can explain why wetting time decreases with Bi additions at
constant wetting test temperature. Trend for wetting force
(Fig.14b) is slightly different. Wetting force increases with
addition of Bi up to about 3%. When the Bi content
increases further (Bi>3%) maximum wetting force drops to
lower values. In other words, the optimum Bi content to
improve wetting force is about 3 wt%. As shown in Fig. 14b
the maximum wetting force follows a parabolic diagram
with highest wetting force around 3% Bi content.
Figure 13. Wetting force balance curves of the alloys at
265°C
(a)
Figure 15. SEM picture showing the bend area of the wire
where tin whisker growth is examined.
(b)
Figure 14. (a) Variation of zero cross wetting time, and (b)
maximum wetting force with Bi content. Wetting time (t0)
decreases with addition of Bi content in all concentrations
studied, whereas wetting force shows a peak around 3% Bi.
SAC305 (2600hr)
Tin whisker
For tin whisker evaluation the alloy with optimum
properties that is SAC-3Bi was evaluated against reference
SAC305. Fig. 15 shows SEM image of the bend area of wire
where observation of whiskers was carried out. Tin whiskers
are observed in SAC305 samples soaked in testing chamber
for 2600hr and 3100hr (see Fig. 16). However, there is no
obvious trace of whiskers on SAC-3Bi sample after 2600hr
and 3100hr soaking. Nevertheless, some signs of whiskers
nucleation may be realized for SAC-3Bi sample soaked for
3100hr (Fig.16). The results show that whisker growth is
effectively suppressed by Bi addition which is consistent
with previously published references [10, 11]. Mitigation of
tin whisker growth with addition of Bi is attributed refining
of grain size structure and also suppressing intermetallic
growth at grain boundaries allowing grain boundary
migration and releasing compressive stresses. In this work
we did not perform detail microstructure study to verify the
suggested mechanism for mitigation of tin whisker growth.
SAC305 (3100hr)
SAC-3Bi (2600hr)
additional strengthening mechanism; precipitation
hardening that is activated at high Bi contents
(Bi>3%).
 Wetting time decreases with Bi additions.
Optimum wetting performance (short wetting time
and high wetting force) is obtained for the alloy
with 3% Bi.
 Tin whisker growth is mitigated in SAC-Bi alloy as
compared with SAC305.
Based on the soldering and mechanical data in this study, it
is concluded that SAC0607-3Bi is the best choice in terms
of melting, wetting, mechanical and reliability performance
as compared to SAC305.
Future work will include the manufacturability, process
capability and product reliability of SAC0607-3Bi for
electronics assembly.
ACKNOWLEDGMENT
The authors would like to thank Dr. Amir Nobari for the
thermodynamic calculation.
SAC-3Bi (3100hr)
Figure 16. SEM surface evaluation of copper wire coated
with SAC305 and SAC-3Bi after 2600 hr. and 3100hr in
60°C and 85% RH chamber.
SUMMARY
A systematic study on low silver SAC0607-Bi solder alloys
was carried out in order to propose an alloy with optimum
soldering and mechaincal properties. Properties such as
thermal behavior, wetting performance, hardness and tensile
strength, as well as tin whisker growth were investigated
and compared with reference SAC305.
The following conclusions can be summarized:
 Alloys with Bi contents higher than 6% form a
detectable low melting temperature phase
 Melting temperature of SAC-Bi alloy decreases
linearly with increasing Bi content up to 3%.
 Pasty range which the difference between solidus
and liquidus temperatures increases drastically with
Bi content above 3%.
 Tensile strength and hardness both increase
linearly with Bi content in the range of 0-6 wt%.
 Tensile elongation decreases with Bi addition. The
trend is more rapid in the alloy with high
concentration of Bi, i.e. 6%. This is attributed to
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